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Leakage current behavior of La-doped Bi2Ti2O7 thin films by a chemical solution deposition method
Materials Letters 58 (2004) 3725 – 3728 www.elsevier.com/locate/matlet
Leakage current behavior of La-doped Bi2Ti2O7 thin films by a chemical solution deposition method Xuena Yanga,*, Baibiao Huanga, Hongbin Wangb, Shuxia Shangc, Weifeng Yaoa, Jiyong Weia a
State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, People’s Republic of China b Shandong Institute of Light Industry, Jinan 250100, People’s Republic of China c Department of Environment Engineering, Shandong University, Jinan 250100, People’s Republic of China Received 9 June 2004; accepted 1 August 2004 Available online 17 August 2004
Abstract Lanthanum-doped Bi2Ti2O7 thin films have been prepared using chemical solution deposition method, and then treated by different annealing temperatures. X-ray diffraction analysis confirmed that the crystallinity of the films increased with increasing annealing temperature and the optimum temperature was found to be 800 8C. The effects of relaxation time and various annealing temperatures and films thickness on the leakage current density were investigated. The results showed that the film annealed at 800 8C had good insulating properties and can be considered using in advanced MOS transistors. D 2004 Elsevier B.V. All rights reserved. Keywords: Thin film; X-ray diffraction; Leakage current density
1. Introduction Bi2Ti2O7 belongs to a family of A2B2O7 compounds with A23+B24+O7 pyrochlore structure. The A2B2O7 formula is often expressed as A2OVd B2O6 to emphasize the interpenetrating networks of a cuprite-like A2OVtetrahedral net with hexagonal tungsten bronze sheets of corner sharing octahedral with B2O6 composition. Pyrochlore structures have a long history and have been reviewed extensively [1]. However, pyrochlore-type bismuth titanate has a somewhat checkered history. The structures of Bi2Ti2O7 were comprised of Ti–O octahedra units and Bi–O tetrahedra units. Melt grown Bi2Ti2O7 single crystals with a doubled cell (a=20.68 2) [2] were later shown [3] to be Bi1.61Zn0.18Ti1.94V0.06O6.62 (a=10.33 2) plus impurities. Radosavljevic et al. [4] produced nearly phase pure Bi1.74Ti2O6.62 at 600 8C using precursors coprecipitated from titanium butoxide and bismuth nitrate. Jiang [5] reported that Bi2Ti2O7 phase disappears from the XRD pattern as annealed at above * Corresponding author. Tel.: +86 5318366324; fax: +86 5318565403. E-mail address: [email protected] (X. Yang). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.08.003
600 8C, suggesting that the phase of Bi2Ti2O7 is unstable at high-annealed temperature (N600 8C). Polycrystalline thin films of composition Bi2Ti2O7 [6] have high permittivity and low leakage current. They have been used to improve the electrical properties of Bi4Ti3O12 [6], PZT [7] and PST [8] ferroelectric thin films and considered promising gate materials for advanced MOS transistors [9]. Recently, Yao et al. [10] has reported that the crystal of Bi2Ti2O7 can be used as photocatalyst, which exhibits higher photocatalytic activity than the Degussa P-25. In view of these promising applications, further detail of the temperature stability and insulating properties of materials with these compositions is timely. In this paper, we report the preparation of the synthesis of Bi2Ti2O7 films with Lanthanum (La)-substitutions on Si h100i substrates and study some insulating properties of the films.
2. Experimentals The precursor solutions were prepared by chemical solution deposition method (CSD). The process steps are
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as following: Bismuth Nitrate [Bi(NO 3)3d 5H2O] and appropriate lanthanum Nitrate[La(NO3)3d nH2O] were initially dissolved in Acetic Acid Glacial. The molar ration of Bi/La is 4:1. After the mixtures were completely dissolved and cooled to the room temperature, proper Acetylacetone and Tetrabutyl titanate were added to the above solution, respectively. The solution was diluted with Ethylene Glycol Monomethyl to adjust its viscosity and surface tension, and then filtered through a 0.2-Am syringe filter to remove dusts and other suspended particles. This is the La/Bi2Ti2O7 (BLTO) precursor solution of 0.1 M. The solution was coated onto Si h100i substrate at 3200 rpm for 30 s to form a single layer. Each layer was heated in air at 300 8C for 10 min to remove solvents and residual organics. The deposited films were finally annealed in air at 550–800 8C for 30 min. The structural properties of the films were studied by means of X-ray diffraction. The X-ray diffraction patterns were recorded on a Rigaku Powder Diffracted Meter using Cu Ka radiation. The insulating properties of the films were measured using a PA-meter/DC Voltage Source (Hp4140B).
Fig. 2. Change of current density with time after applying a bias voltage of 5 V.
Fig. 1 shows the XRD results of the BLTO films annealed at various indicated temperatures for 30 min. The film annealed at 550 8C is amorphous. As the annealing temperature increases to 600 8C, the film begins to crystallize. When the annealing temperature increased to 800 8C, sharp (111) peak can be observed, which indicate that the BLTO films crystallize very well. The d values and intensities of the peaks agree very well with those given in JCPDS data cards for Bi2Ti2O7, suggesting that the films were strong (111) orientation. The increase of peak intensities and decrease of full widths at half maximum with increasing annealing temperature indicate the increase
of grain size. The above results indicate that the La-doped Bi2Ti2O7 films are more stable than the Bi2Ti2O7 films without La modification [12]. In order to study the films’ insulating properties, the samples were prepared for MFSM (Metal/Film/Semiconductor/Metal) configurations with diameters of 1 mm gold dots on the top of the films fabricated by Direct Current Sputtering. The bottom electrode of the films was coated completely by Au. The insulating properties of the films were investigated in terms of Leakage Current Density. The results are as follows. Fig. 2 shows the current–time (I–t) characteristics of 0.6 Am thick BLTO films with 5 V of bias voltage. The current density decreased rapidly during the first 30 s after the application of bias voltage and reached the steady state of 5.87109A/cm2. The decrease of current with time is caused by the dielectric relaxation. Its contribution can be neglected when the measuring time is large enough. The following measurements were made at 30 s after the application of bias voltage.
Fig. 1. XRD patterns of BLTO films annealed at different temparature.
Fig. 3. Changes of leakage current density of BLTO films with different annealing temparature.
3. Results and discussion
X. Yang et al. / Materials Letters 58 (2004) 3725–3728
Fig. 4. J–E characteristics of BLTO film with different film thicknesses.
Fig. 3 shows I–V characteristics as function of temperature ranging from 600 to 800 8C. The evolution of the I–V characteristics with applied voltage indicates that different conduction mechanisms occur depending on annealing temperature. The leakage current mechanisms commonly reported in dielectric thin films include ohmic conduction (usually at low field), space-charge-limited conduction (SCLC), Poole-Frenkel conduction, and Schottky conduction [11]. For the film annealed at 600 8C, the current presents a higher variation with applied voltage than for higher annealing temperature. At low voltage (below 1.5 V), current I of the film annealed at 600 8C was linearly dependent on voltage V, i.e. the I–V curve exhibited ohmic behavior. This ohmic mode occurs in insulating film as long as the film is quasineutral, that is, as long as the bulk generated current in the film exceeds the current due to injected free carriers from the electrode. At higher fields (from 1.5 to 2.5 V), the nonlinear I–V relationship could be observed. In this region, the injection of charge carriers in the bulk of the film takes place. For stronger injection, the insulator traps fill up and a space charge appears. A square law region appeared above 1.5 V indicating onset of the space-charge-limited conduction. As the applied voltage is increased further (above 2.5 V), very strong injection of charge carriers takes place, and causes an increase in the density of filled trapping sites. If sufficient charge is injected into the insulator, all the traps will become filled. Further injected charge then exists as free charge in the conduction band and contributes to the current. According to the Fig. 3, it is found that at the applied bias voltage of 3 V, the leakage current density of the films annealed at 600, 700, 750 and 800 8C are 3.77107, 3.1108, 1.75108 and 2.01109 A/cm2, respectively. The film annealed at 800 8C exhibited a ` d cm when a bias resistivity in the range of 1014–1013 U voltage of 0–5 V applied to the capacitor. The data above indicated that the BLTO film annealed at 800 8C had good insulating properties and resistance to breakdown. It is of great importance for use in advanced MOS transistors.
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Fig. 4 shows the leakage current density of the BLTO films with different film thicknesses. The annealing temperature of the samples was all 800 8C for 30 min. The insulating properties of the samples were found to be dependent on the film thicknesses. When the BLTO film thickness increases, the polycrystalline nature of the film could induce stress, which can generate new interface states, with the result that the leakage current density increased. According to Fig. 4, it is found that the leakage current density of the 950-nm thick film was the largest than the others at the same applied voltage. When the applied voltage is more than 3.5 V, the 950nm thick film increases sharply and tends to break down. However, the leakage current density of the other three samples increase gradually with increasing applied voltage, which indicated that the films with lower thickness had good insulating properties and resistance to break down.
4. Conclusions Thin films of La-doped Bi2Ti2O7 were prepared on Si h100i substrates by chemical solution decomposition method. The BLTO film began to crystallize at an annealing at 600 8C. The sample annealed at 800 8C for 30 min had good crystallization, which indicated that the La-doped Bi2Ti2O7 films are more stable than the Bi2Ti2O7 films without La modification. The I–V characteristics showed ohmic conductivity in the lower voltage range and spacecharge-limited conductivity at higher field. The film annealed at 800 8C showed a resistivity in the range of 1014–1013 Vd cm when a bias voltage of 0–5 V applied. The curves of leakage current density versus applied voltage with different film thicknesses showed that the films with lower thickness had good insulating properties and resistance to break down.
Acknowledgements We acknowledge the support of the National Natural Science Foundation of China under Grant No. 05130390.
References [1] M.A. Subramanian, G. Aravamudan, G.V. Subba Rao, Prog. Solid State Chem. 15 (1983) 55 – 143. [2] S. Shimada, K. Kodaira, T. Matshushita, J. Cryst. Growth 41 (1977) 317 – 320. [3] V. Kahlenberg, H. Bohm, J. Alloys Compd. 223 (1995) 142 – 146. [4] I. Radosavljevic, J.S.O. Evans, A.W. Sleight, J. Solid State Chem. 136 (1998) 63 – 66. [5] A.Q. Jiang, Z.X. Xu, L.D. Zhang, Appl. Phys. Lett. 74 (1999) 114 – 116.
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[6] S.W. Wang, W. Lu, N. Li, Z.F. Li, H. Wang, M. Wang, X.C. Shen, Mater. Res. Bull. 37 (2002) 1691 – 1697. [7] S.W. Wang, H. Wang, S. Shang, J. Huang, Z. Wang, M. Wang, J. Cryst. Growth 217 (2000) 388 – 392. [8] F.Y. Jiang, R.C. Liu, J. Cryst. Growth 263 (2004) 385 – 388. [9] S.W. Wang, H. Wang, X.M. Wu, S.X. Shang, M. Wang, Z.F. Li, J. Cryst. Growth 224 (2001) 323 – 326.
[10] W.F. Yao, H. Wang, X.H. Xu, J.T. Zhou, X.N. Yang, Y. Zhang, Sh. X. Shang, Appl. Catal., A Gen. 259 (2004) 29 – 33. [11] X.N. Shen, H. Wang, J. Funct. Mater. 27 (1996) 295 – 301 (Chinese). [12] H. Wang, L.W. Fu, S.X. Shang, X.L. Wang, M.H. Jiang, J.phys.D: Appl. Phys. 27 (1994) 393 – 395.
Leakage current behavior of La-doped Bi2Ti2O7 thin films by a chemical solution deposition method Xuena Yanga,*, Baibiao Huanga, Hongbin Wangb, Shuxia Shangc, Weifeng Yaoa, Jiyong Weia a
State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, People’s Republic of China b Shandong Institute of Light Industry, Jinan 250100, People’s Republic of China c Department of Environment Engineering, Shandong University, Jinan 250100, People’s Republic of China Received 9 June 2004; accepted 1 August 2004 Available online 17 August 2004
Abstract Lanthanum-doped Bi2Ti2O7 thin films have been prepared using chemical solution deposition method, and then treated by different annealing temperatures. X-ray diffraction analysis confirmed that the crystallinity of the films increased with increasing annealing temperature and the optimum temperature was found to be 800 8C. The effects of relaxation time and various annealing temperatures and films thickness on the leakage current density were investigated. The results showed that the film annealed at 800 8C had good insulating properties and can be considered using in advanced MOS transistors. D 2004 Elsevier B.V. All rights reserved. Keywords: Thin film; X-ray diffraction; Leakage current density
1. Introduction Bi2Ti2O7 belongs to a family of A2B2O7 compounds with A23+B24+O7 pyrochlore structure. The A2B2O7 formula is often expressed as A2OVd B2O6 to emphasize the interpenetrating networks of a cuprite-like A2OVtetrahedral net with hexagonal tungsten bronze sheets of corner sharing octahedral with B2O6 composition. Pyrochlore structures have a long history and have been reviewed extensively [1]. However, pyrochlore-type bismuth titanate has a somewhat checkered history. The structures of Bi2Ti2O7 were comprised of Ti–O octahedra units and Bi–O tetrahedra units. Melt grown Bi2Ti2O7 single crystals with a doubled cell (a=20.68 2) [2] were later shown [3] to be Bi1.61Zn0.18Ti1.94V0.06O6.62 (a=10.33 2) plus impurities. Radosavljevic et al. [4] produced nearly phase pure Bi1.74Ti2O6.62 at 600 8C using precursors coprecipitated from titanium butoxide and bismuth nitrate. Jiang [5] reported that Bi2Ti2O7 phase disappears from the XRD pattern as annealed at above * Corresponding author. Tel.: +86 5318366324; fax: +86 5318565403. E-mail address: [email protected] (X. Yang). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.08.003
600 8C, suggesting that the phase of Bi2Ti2O7 is unstable at high-annealed temperature (N600 8C). Polycrystalline thin films of composition Bi2Ti2O7 [6] have high permittivity and low leakage current. They have been used to improve the electrical properties of Bi4Ti3O12 [6], PZT [7] and PST [8] ferroelectric thin films and considered promising gate materials for advanced MOS transistors [9]. Recently, Yao et al. [10] has reported that the crystal of Bi2Ti2O7 can be used as photocatalyst, which exhibits higher photocatalytic activity than the Degussa P-25. In view of these promising applications, further detail of the temperature stability and insulating properties of materials with these compositions is timely. In this paper, we report the preparation of the synthesis of Bi2Ti2O7 films with Lanthanum (La)-substitutions on Si h100i substrates and study some insulating properties of the films.
2. Experimentals The precursor solutions were prepared by chemical solution deposition method (CSD). The process steps are
3726
X. Yang et al. / Materials Letters 58 (2004) 3725–3728
as following: Bismuth Nitrate [Bi(NO 3)3d 5H2O] and appropriate lanthanum Nitrate[La(NO3)3d nH2O] were initially dissolved in Acetic Acid Glacial. The molar ration of Bi/La is 4:1. After the mixtures were completely dissolved and cooled to the room temperature, proper Acetylacetone and Tetrabutyl titanate were added to the above solution, respectively. The solution was diluted with Ethylene Glycol Monomethyl to adjust its viscosity and surface tension, and then filtered through a 0.2-Am syringe filter to remove dusts and other suspended particles. This is the La/Bi2Ti2O7 (BLTO) precursor solution of 0.1 M. The solution was coated onto Si h100i substrate at 3200 rpm for 30 s to form a single layer. Each layer was heated in air at 300 8C for 10 min to remove solvents and residual organics. The deposited films were finally annealed in air at 550–800 8C for 30 min. The structural properties of the films were studied by means of X-ray diffraction. The X-ray diffraction patterns were recorded on a Rigaku Powder Diffracted Meter using Cu Ka radiation. The insulating properties of the films were measured using a PA-meter/DC Voltage Source (Hp4140B).
Fig. 2. Change of current density with time after applying a bias voltage of 5 V.
Fig. 1 shows the XRD results of the BLTO films annealed at various indicated temperatures for 30 min. The film annealed at 550 8C is amorphous. As the annealing temperature increases to 600 8C, the film begins to crystallize. When the annealing temperature increased to 800 8C, sharp (111) peak can be observed, which indicate that the BLTO films crystallize very well. The d values and intensities of the peaks agree very well with those given in JCPDS data cards for Bi2Ti2O7, suggesting that the films were strong (111) orientation. The increase of peak intensities and decrease of full widths at half maximum with increasing annealing temperature indicate the increase
of grain size. The above results indicate that the La-doped Bi2Ti2O7 films are more stable than the Bi2Ti2O7 films without La modification [12]. In order to study the films’ insulating properties, the samples were prepared for MFSM (Metal/Film/Semiconductor/Metal) configurations with diameters of 1 mm gold dots on the top of the films fabricated by Direct Current Sputtering. The bottom electrode of the films was coated completely by Au. The insulating properties of the films were investigated in terms of Leakage Current Density. The results are as follows. Fig. 2 shows the current–time (I–t) characteristics of 0.6 Am thick BLTO films with 5 V of bias voltage. The current density decreased rapidly during the first 30 s after the application of bias voltage and reached the steady state of 5.87109A/cm2. The decrease of current with time is caused by the dielectric relaxation. Its contribution can be neglected when the measuring time is large enough. The following measurements were made at 30 s after the application of bias voltage.
Fig. 1. XRD patterns of BLTO films annealed at different temparature.
Fig. 3. Changes of leakage current density of BLTO films with different annealing temparature.
3. Results and discussion
X. Yang et al. / Materials Letters 58 (2004) 3725–3728
Fig. 4. J–E characteristics of BLTO film with different film thicknesses.
Fig. 3 shows I–V characteristics as function of temperature ranging from 600 to 800 8C. The evolution of the I–V characteristics with applied voltage indicates that different conduction mechanisms occur depending on annealing temperature. The leakage current mechanisms commonly reported in dielectric thin films include ohmic conduction (usually at low field), space-charge-limited conduction (SCLC), Poole-Frenkel conduction, and Schottky conduction [11]. For the film annealed at 600 8C, the current presents a higher variation with applied voltage than for higher annealing temperature. At low voltage (below 1.5 V), current I of the film annealed at 600 8C was linearly dependent on voltage V, i.e. the I–V curve exhibited ohmic behavior. This ohmic mode occurs in insulating film as long as the film is quasineutral, that is, as long as the bulk generated current in the film exceeds the current due to injected free carriers from the electrode. At higher fields (from 1.5 to 2.5 V), the nonlinear I–V relationship could be observed. In this region, the injection of charge carriers in the bulk of the film takes place. For stronger injection, the insulator traps fill up and a space charge appears. A square law region appeared above 1.5 V indicating onset of the space-charge-limited conduction. As the applied voltage is increased further (above 2.5 V), very strong injection of charge carriers takes place, and causes an increase in the density of filled trapping sites. If sufficient charge is injected into the insulator, all the traps will become filled. Further injected charge then exists as free charge in the conduction band and contributes to the current. According to the Fig. 3, it is found that at the applied bias voltage of 3 V, the leakage current density of the films annealed at 600, 700, 750 and 800 8C are 3.77107, 3.1108, 1.75108 and 2.01109 A/cm2, respectively. The film annealed at 800 8C exhibited a ` d cm when a bias resistivity in the range of 1014–1013 U voltage of 0–5 V applied to the capacitor. The data above indicated that the BLTO film annealed at 800 8C had good insulating properties and resistance to breakdown. It is of great importance for use in advanced MOS transistors.
3727
Fig. 4 shows the leakage current density of the BLTO films with different film thicknesses. The annealing temperature of the samples was all 800 8C for 30 min. The insulating properties of the samples were found to be dependent on the film thicknesses. When the BLTO film thickness increases, the polycrystalline nature of the film could induce stress, which can generate new interface states, with the result that the leakage current density increased. According to Fig. 4, it is found that the leakage current density of the 950-nm thick film was the largest than the others at the same applied voltage. When the applied voltage is more than 3.5 V, the 950nm thick film increases sharply and tends to break down. However, the leakage current density of the other three samples increase gradually with increasing applied voltage, which indicated that the films with lower thickness had good insulating properties and resistance to break down.
4. Conclusions Thin films of La-doped Bi2Ti2O7 were prepared on Si h100i substrates by chemical solution decomposition method. The BLTO film began to crystallize at an annealing at 600 8C. The sample annealed at 800 8C for 30 min had good crystallization, which indicated that the La-doped Bi2Ti2O7 films are more stable than the Bi2Ti2O7 films without La modification. The I–V characteristics showed ohmic conductivity in the lower voltage range and spacecharge-limited conductivity at higher field. The film annealed at 800 8C showed a resistivity in the range of 1014–1013 Vd cm when a bias voltage of 0–5 V applied. The curves of leakage current density versus applied voltage with different film thicknesses showed that the films with lower thickness had good insulating properties and resistance to break down.
Acknowledgements We acknowledge the support of the National Natural Science Foundation of China under Grant No. 05130390.
References [1] M.A. Subramanian, G. Aravamudan, G.V. Subba Rao, Prog. Solid State Chem. 15 (1983) 55 – 143. [2] S. Shimada, K. Kodaira, T. Matshushita, J. Cryst. Growth 41 (1977) 317 – 320. [3] V. Kahlenberg, H. Bohm, J. Alloys Compd. 223 (1995) 142 – 146. [4] I. Radosavljevic, J.S.O. Evans, A.W. Sleight, J. Solid State Chem. 136 (1998) 63 – 66. [5] A.Q. Jiang, Z.X. Xu, L.D. Zhang, Appl. Phys. Lett. 74 (1999) 114 – 116.
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X. Yang et al. / Materials Letters 58 (2004) 3725–3728
[6] S.W. Wang, W. Lu, N. Li, Z.F. Li, H. Wang, M. Wang, X.C. Shen, Mater. Res. Bull. 37 (2002) 1691 – 1697. [7] S.W. Wang, H. Wang, S. Shang, J. Huang, Z. Wang, M. Wang, J. Cryst. Growth 217 (2000) 388 – 392. [8] F.Y. Jiang, R.C. Liu, J. Cryst. Growth 263 (2004) 385 – 388. [9] S.W. Wang, H. Wang, X.M. Wu, S.X. Shang, M. Wang, Z.F. Li, J. Cryst. Growth 224 (2001) 323 – 326.
[10] W.F. Yao, H. Wang, X.H. Xu, J.T. Zhou, X.N. Yang, Y. Zhang, Sh. X. Shang, Appl. Catal., A Gen. 259 (2004) 29 – 33. [11] X.N. Shen, H. Wang, J. Funct. Mater. 27 (1996) 295 – 301 (Chinese). [12] H. Wang, L.W. Fu, S.X. Shang, X.L. Wang, M.H. Jiang, J.phys.D: Appl. Phys. 27 (1994) 393 – 395.